1. Field of the Invention
The present invention relates to a mechanism to allow a plurality of different communication systems respectively having different communication modes to coexist with one another on a single communication medium. More particularly, the present invention relates to a technique of allowing Power Line Communication (PLC) modems, one of which ensures QoS (Quality of Service) required by communication service and another of which performs best-effort communication, to coexist with each other, and also relates to a communication apparatus included in each of the plurality of the communication systems.
2. Description of the Background Art
There has been a PLC technique as one of communication means for connecting a home personal computer (PC) to network equipment such as a broadband router so as to access the Internet from the home PC. The PLC technique utilizes an existing power line as a communication medium, and thus can realize high-speed communication only by inserting AC mains plug into any one of AC mains outlets in a household without additional wiring. Therefore, research and development or an experimental demonstration relating to the PLC technique is being performed vigorously worldwide. In Europe and the United States, there are a number of cases where the PLC technique has already been commercialized.
An exemplary PLC technique is the HomePlug Ver.1.0 specified by HomePlug Powerline Alliance in the U.S. For example, see p54-p63 of “A Comparative Performance Study of Wireless and Power Line Networks”, written by Yu-Ju Lin et. al., published by, IEEE Communication Magazine, in April 2003. This specification considers the Internet, an e-mail, and file transfer by using a PC as main use applications thereof. In the specification, a CSMA/CA method is adopted as a media access control so as to control PLC modems accessing a power line. Therefore, the specification can realize only best-effort communication which cannot ensure a fixed band width to be used.
FIG. 7 is a diagram illustrating a general configuration of the Internet access from the home PC.
A PC 701 used by a user is connected to an Internet access router 704 via the Ethernet (registered trademark) 702, and then connected to the Internet 705 via an access line 703. Generally, an ADSL (Asymmetric Digital Subscriber Line) or an FTTH (Fiber To The Home) is used as an exemplary access line 703. In most cases, in the household, a place where the access line 703 is drawn is different from a room in which the PC 701 is situated. In such case, there is a problem in that the Ethernet (registered trademark) 702 cable has to be routed from the Internet access router 704 to the PC 701.
In the field of the PLC, the conversion adapter between the powerline and the Ethernet (registered trademark) (hereinafter referred to as a P/E conversion adapter) has been commercialized so as to reduce the routing. FIG. 8 shows a general configuration relating to the Internet access in the case where the P/E conversion adapter is used.
In FIG. 8, two P/E conversion adapters 805 are respectively connected to the AC mains outlets in one room where a PC 801 and situated and in another room where an Internet access router 804 is situated. The best-effort communication is realized by using the PLC via a power line 807 and a distribution switch board 808 in the household. In this manner, with the use of the PLC, it is possible to realize the high-speed communication only by inserting the AC mains plug into the AC mains outlets situated anywhere in the household without the additional wiring.
On the other hand, there is a move to build a new network appliance system which applies the PC-led Internet technology to AV equipment and communication equipment. The move is developing into a new system such as a linkage between an AV server (a DVD recorder, a HDD recorder and the like) and a TV, which are respectively situated indifferent rooms, or a combination between an IP phone or an IP camera, which applies the Internet technology, and the TV or the PC.
FIG. 9 is a diagram showing an exemplary new system as above described. Unlike the conventional Internet, e-mail and file transfer, real time communication needs to be ensured for an AV stream and voice communication. Particularly, in the case of a telephone where mutual voice communication is taken place in real time, there is a strong request for “no” communication delay, and thus the communication delay is restricted to as little as about 10 msecs., in general. For such service that needs to ensure QoS (Quality of Service), there has been a problem in that the best-effort communication cannot satisfy the requested quality.
To ensure the QoS, a PLC method is being developed. For example, see “A Media Access Control Method for High-Speed Power Line Communication System Modems”, written by Shinichiro Ohmi, IEEE CCNC 2004. FIG. 10 is a diagram illustrating a PLC which performs a best-effort communication (hereinafter referred to as a best-effort type PLC) and a PLC which needs to ensure QoS (hereinafter referred to as a QoS type PLC). In FIG. 10, the vertical axis indicates a frequency, and the horizontal axis indicates time.
In the case of HomePlug ver.1.0, which is an exemplary best-effort type PLC, the frequency to be used approximately ranges from 2 MHz to 21 MHz. The time axis varies depending on a data generation timing or a data amount. In the case of displaying an Internet website or obtaining an e-mail, a service thereof may be acceptable even if the display or obtainment is delayed, as long as the delay is within an acceptable time range.
On the other hand, the QoS type PLC is, in most cases, aimed at a high-speed transmission of a video data, and uses a broader frequency range. Further, in order to ensure the QoS, one QoS controller is situated in a system. The QoS controller controls timing and an amount of data transmitted from the power line modem by transmitting a beacon at fixed intervals ((b) of FIG. 10). A function of the QoS controller may be included in one power line modem, and in an example shown in FIG. 9, the Qos controller is mounted in a P/E conversion adapter 909.
Assuming that an amount of video data as well as a communication speed are respectively fixed, data is transmitted to the power line for a fixed time period at fixed time intervals ((b) of FIG. 10). If the data does not arrive at a receiving side by a predetermined time, video distortion will be induced, and consequently the service cannot be provided. Further, since equipment connected to the power line and an operation state thereof varies overtime, a communication condition thereof is not constant, but substantially varies from hour to hour. When the communication speed slows down, time required for transmitting the data varies. Therefore, when a power line modem which transmits video data detects a slow down in the transmission speed, the power line modem informs the QoS controller of the slow down by using a communication command, and also receives a time allocation required for transmitting the same amount of data as before the slow down, thereby ensuring the QoS. This situation is shown in FIG. 11. In FIG. 11, the power line modem, which has detected the slow down in the transmission speed, transmits a change allocated time command to the QoS controller so as to change and extend the data transmission time thereafter. Accordingly, it is possible to keep transmitting a fixed amount of data per unit time.
In this manner, various types of PLC techniques have been developed. The power lines wired into respective households are each connected to the distribution switch board. In the case where different types of power line modems are used in one household, from a standpoint of one type of a power line modem, a signal transmitted to the power line from another type of a power line modem is merely seen as a noise. Therefore, in this case, when communications are performed simultaneously, as shown in (c) of FIG. 10, the communications are interrupted mutually, or the respective communications become incapable, whereby the communication speed slows down significantly.
To solve this problem, Japanese Laid-Open Patent Publication No. 2002-368831, for example, suggests a method of controlling data transmission of power line modems, in the case where there is, on a single power line, a plurality of the power line modems respectively having different data communication modes. FIG. 12 is a diagram illustrating this conventional technique.
In FIG. 12, a selection section 61 in an administrative processor 6 selects, for example, mode B power line modems 4a to 4m as transmission permitted power line modems. A message generation section 62 generates a transmission permission message which instructs the mode B power line modems 4a to 4m of transmission permission, and also generates a transmission prohibition message which instructs mode A power line modems 3a to 3m of transmission prohibition. A mode A power line modem 3n transmits the transmission prohibition message to the mode A power line modems 3a to 3m. A mode B power line modem 4n transmits the transmission permission message to the mode B powerline modems 4a to 4m. 
In the above-described technique, however, all possible communication modes need to be mounted on a coexistence control terminal, and thus in the case where there exist three or more communication modes, it causes a problem since equipment costs increases. Further, in the case of emergence of a novel communication mode in the future, a coexistence control terminal accommodating the novel communication mode needs to be developed, which consequently leads to a problem in that it is very difficult to maintain a state of the coexistence.
As an exemplary technique to solve the problem, coexistence control signal transmitting section, which is simple and easily mountable, is provided to all the PLC systems, and a communication band is processed through TDM (Time Division Multiplexing) by using the coexistence control signal so as to enhance the coexistence of a plurality of different communication modes.
For example, as shown in FIG. 13, the communication band is defined such that a time period, during which a coexistence control signal 1301 is sent and received at a predetermined timing, is repeated in a cycle 1309, and a time period 1308 are divided by a time period 1307 which is constituted of a unit of N segments (communication slots). By using the coexistence control signal 1301 sent and received in the cycle 1309, the right of use of the N communication slots is allocated to any one of the PLC systems individually, whereby coexistence among the communication systems can be realized.
To enhance economical medium sharing among communication systems, it is desirable to set an exclusive medium use rate minutely. However, in the conventional communication system shown in FIG. 13, the communication slots, which are fixed with respect to the respective time periods 1307, are allocated to respective communication systems, and thus the exclusive medium use rate of each of the communication systems can be set only in a unit of 1/N.
In order to minutely set the exclusive medium use rate, a method of increasing the number of the communication slots dividing the time period 1307 (that is, incrementing a variable N) maybe considered. However, each of the communication slots needs a header part of a transmission frame, and thus the more the number of the communication slots increases, the more transmission efficiency deteriorates since overhead of the header part increases. For example, in the case where communication is performed by using a popular PLC system of 10 Mbps or more, it is preferable to set one communication slot length to 3 msecs. or longer.
On the other hand, in order to minutely set the exclusive medium use rate, another method for extending the time period 1307 (in this case as well, the variable N is incremented), while keeping the one communication slot length as it is, may be considered. However, since the TDM cycle are extended, it is difficult for this method to be applied to a service which requires “low” latency such as a VoIP (Voice over IP). Specifically, the TDM cycle needs to be 20 to 30 msecs., and it is only possible to set about N=10 even for the case of the minimum communication slot length.
Therefore, it is difficult to satisfy the QoS requirement and minutely set the exclusive medium use rate by using only the conventional simple configuration, as above described.